1. Field of the Invention
This invention relates to an hydraulically actuated friction element such as a clutch or brake for use in an automatic transmission of a motor vehicle.
2. Description of the Prior Art
Ball check valves are used in the hydraulic circuit of an automatic transmission to permit one-way flow of hydraulic fluid to friction elements that control operation of planetary gear units. These check valves are susceptible to blockage caused by contaminants entrained in the hydraulic fluid supplied to the check valve.
It is conventional practice to include a rather coarse filter located in the sump of the transmission, through which filter hydraulic fluid passes as it is drawn from the sump by a hydraulic pump that supplies pressurized hydraulic fluid to the control and actuation system of the transmission. Despite the use of hydraulic filters located in the sump, correct actuation of hydraulic clutches and brakes is compromised when contaminants carried by the hydraulic fluid adversely affect operation of the ball check valves associated with the friction clutches and brakes. Attempts have been made to attach a filter onto a cartridge ball check assembly by a snap fit or press fit or by staking the filter directing to a clutch or brake piston. Snap fitting or staking a filter requires additional complexity, an additional manufacturing operation, and increase in cost.
It is preferable to locate a fluid filter directly over the cartridge assembly that contains the ball check valve so that the filter blocks contaminant present in the hydraulic fluid near the hydraulic clutch or brake. The contaminants are filtered before reaching the cartridge assembly and before they affect adversely correct operation of the check valve.